Field of the Invention
The present invention relates to a magnetic proximity sensor for detecting a position of a detected body in non-contact.
Description of the Related Art
This type of proximity sensor causes an increasing interest as a sensor that is excellent in a point that there does not exist degradation of the contact point due to contact that is made in a so-called contact point type switch. Therefore, for example, as an application of this type of proximity sensor to a vehicle, a pedal sensor, a door opening/closing sensor, a hood opening/closing sensor, a seat position sensor, a seat belt strapping sensor, and the like are under consideration. For example, Japanese Patent No. 4066716 discloses the conventional proximity sensor as follows. This proximity sensor comprises at least two opposing permanent magnets (hereinafter, called simply a magnet), a first yoke interposed between magnetic pole faces thereof, a protruding portion provided in the first yoke to extend in a vertical direction from an intermediate position between the magnets, a second yoke separated from the protruding portion, and a magnetic detection element arranged in a gap between the protruding portion and the second yoke. In addition, the two magnets are designed to have the same magnetic pole direction, and therefore N pole of one magnet opposes S pole of the other magnet to put the first yoke therebetween.
As a result, a position of a detected member made of a magnetic body moving in parallel with a projecting direction of the protruding portion is detected in a state of being separated from the magnets having the first yoke therebetween, the second yoke, and the like. That is, a magnetic line of one of the two magnets goes out from N pole, passes through a space, and returns back to S pole from the second yoke via the magnetic detection element and the protruding portion. A magnetic line of the other of the two magnets goes out from N pole, and returns back to S pole via the protruding portion, the magnetic detection element, the second yoke and the space. Therefore in a case where the detected member is not positioned to be close to the sensor, magnetic fluxes of the two magnets are reversed with each other in a region of the magnetic detection element, and the magnetic flux density that is detected by the magnetic detection element becomes zero. On the other hand, in a case where the detected member moves to a close position to the sensor, since the detected member becomes a part of a magnetic circuit of one magnet, most of the magnetic fluxes flow to the magnetic detection element via the detected member. In contrast, since the magnetic circuit of the other magnet has no change, the equilibrium of the magnetic flux in the region of the magnetic detection element is lost, and as a result the magnetic detection element detects a change in the magnetic flux density.
However, in the conventional proximity sensor, at least two magnets are required to be mounted to the first yoke to make the magnetic poles opposing with each other differ, and therefore, for preventing the event that a magnetic contamination substance is attached to the magnet at the time of the assembly of the sensor in a manufacturing line or the like and the equilibrium of the magnetic flux is lost regardless of the detected member is not positioned to be close to the sensor, the strict environment and the process management in the manufacturing line or the like are required.
In addition, since the permanent magnet originally varies a lot in the magnetic force depending on its individual, even if the assembly of the sensor in the manufacturing line is strictly managed, the magnetic fluxes of the two magnets in a region of the magnetic detection element do not necessarily come to equilibrium or change as desired. In fact, the detection position of the detected member varies a lot for each commercial product. However, any adjustment measure is not taken for overcoming this problem, and therefore it is still difficult to put this conventional proximity sensor into practice.